Bake the AO to add Grunge
A UV map is required in order to bake.

To UV unwrap a mesh, switch to edit mode (&#8633; Tab) and press U.
Usually an automated method like Smart project or Lightmap pack is good enough for baking, but for other uses you may want to use manually placed seams.

See 


What does unwrapping a model do and why is it important?
How to properly unwrap my mesh?